Fencing (C-13) · Lesson 3 of 6

Post Embedment Depth: CBC Equation 18-1 and Table 1806.2

"Post Embedment Depth: CBC Equation 18-1 and Table 1806.2" is lesson 3 of 6 in the Fencing (C-13) Learning Hub. It opens with "When the California Formula Applies" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. When the California Formula Applies
  2. The Equation and Its Letters
  3. Getting S1 From Table 1806.2
  4. Where P and h Come From
  5. Solve by Trial and Check the Backfill

The worked example in the course applies this lesson to a realistic job in 4 steps, from "Loads and geometry" through "Soil value".

What you'll be able to do

  • Explain the main idea of "Post Embedment Depth: CBC Equation 18-1 and Table 1806.2" in plain language you can use on the exam.
  • Apply this step with confidence: "Getting S1 From Table 1806.2".
  • Apply this step with confidence: "Where P and h Come From".
  • Apply this step with confidence: "Solve by Trial and Check the Backfill".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Embedded Post Depth: CBC Equation 18-1Guard-type fence post, 200 lb at 42 in., clay soilPOST IN A 12 IN. FOOTINGGRADEP = 200 lbh = 3.5 ft(42 in.)d = 5.0 ftb = 1 ftclay:100 psf/ftSketch not to scaleEQUATION 18-1, BY TRIALd = 0.5A{1 + [1 + (4.36h/A)]1/2}A = 2.34P / (S1 x b)S1 = 100 psf/ft x (d / 3)Try d = 4.0 ft: S1 = 133 psf, A = 3.51d needed = 5.81 ft, more than 4.0FAILTry d = 5.0 ft: S1 = 167 psf, A = 2.81d needed = 4.97 ft, under 5.0PASSd is not over 12 ft in the formula.No one-third increase: not a wind orseismic load. S1 is read at d / 3.RESULT: embed the post 5.0 ft (60 in.) in a 12 in. footingSource: 2025 CBC §1807.3.2.1 Eq. 18-1 and Table 1806.2 (Ch. 18 pp. 8, 15); §1607.9.1 (200 lb); §1015.3 (42 in.).CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: When the California Formula Applies

Standard plans cover chain link, but a wood, steel, or ornamental post that carries a guard load, or any fence an engineer or building official asks you to check, is designed with the building code. 2025 CBC §1807.3 (Embedded Posts and Poles, Chapter 18 pp. 14-15) governs posts or poles embedded in earth or in concrete footings in earth. It gives the depth to resist lateral loads in §1807.3.2, with the limitation (§1807.3.1) that posts embedded in earth are not to give lateral support to plaster, masonry, or concrete unless bracing limits the deflection.

Step 2: The Equation and Its Letters

For a post with no lateral constraint at the ground surface, §1807.3.2.1 gives Equation 18-1: d = 0.5A{1 + [1 + (4.36h/A)]^1/2}, where A = 2.34P/(S1 x b). Here d is the depth of embedment in feet (not over 12 ft for computing lateral pressure), P is the applied lateral force in pounds, h is the distance in feet from the ground surface to the point where P acts, b is the diameter of the round post or footing (or diagonal of a square one) in feet, and S1 is the allowable lateral soil-bearing pressure from §1806.2 taken at a depth of one-third of the embedment depth, in psf.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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